Block copolymers have been increasingly employed in adhesive compositions primarily because of their high cohesive strengths and their ability to "cross-link" without a chemical vulcanization step. These block copolymers, such as those described in Harlan, U.S. Pat. No. 3,239,478, are primarily either linear or radial styrene-butadiene or styrene-isoprene block copolymers. The linear copolymers typically have the structure styrene-butadiene-styrene or styrene-isoprene-styrene. The high cohesive strength of these styrene-diene block copolymers is primarily attributed to their network structure resulting from their well-known domain formation.
However, this very high cohesive strength is often a detrimental quality in certain applications. For example, it is known that in adhesives based on natural rubber or SBR, aggressive tack is inversely related to cohesive strength. That is, as the rubber is crosslinked to a higher degree, the cohesive strength increases but the aggressive tack decreases. Previous patents disclose adhesive compositions containing block copolymers which impart very high cohesive strength to the adhesive. In formulations containing only block copolymer and tackifying resin, aggressive tack is marginal and oil is normally required in the formulation to improve aggressive tack. If some way were found to reduce the cohesive strength of the block copolymer used in an adhesive formulation, substantially improved aggressive tack could be obtained, even in formulations which do not contain oil.
Control of the cohesive strength of an adhesive composition is also important for controlling the peel strength of the adhesive; that is, the force required to remove, for example, a pressure sensitive label from an article on which it has been placed. If some way were found to reduce the cohesive strength of the adhesive composition on the label, the force required to remove the label would be higher because there would exist cohesive failure rather than adhesive failure.